To create multi-colored or white light based on additive color mixing principles, often multiple different sources of colored light are employed, for example red light, blue light and green light, to represent the “primary” colors. These three primary colors roughly represent the respective spectral sensitivities typical of the three different types of cone receptors in the human eye (having peak sensitivities at approximately 650 nanometers for red, 530 nanometers for green, and 425 nanometers for blue) under photopic (i.e., daytime, or relatively bright) viewing conditions. Much research has shown that additive mixtures of primary colors in different proportions can create a wide range of colors discernible to humans. This is the well-known principle on which many color displays are based, in which a red light emitter, a blue light emitter, and a green light emitter are energized in different proportions to create a wide variety of perceivably different colors, as well as white light, based on additive mixing of the primary colors.
A visual stimulus corresponding to a perceivable color can be described in terms of the energy emission of a light source that gives rise to the visual stimulus. A “spectral power distribution” (SPD) of the energy emission from a light source often is expressed as a function of wavelength λ, and provides an indication of an amount of radiant power per small constant-width wavelength interval that is present in the energy emission throughout the visible spectrum. The SPD of energy emission from a light source may be measured via spectroradiometer, spectrophotometer or other suitable instrument. A given visual stimulus may be thought of generally in terms of its overall perceived strength and color, both of which relate to its SPD.
One measure of describing the perceived strength of a visual stimulus, based on the energy emitted from a light source that gives rise to the visual stimulus, is referred to as “luminous intensity,” for which the unit of “candela” is defined. Specifically, the unit of candela is defined such that a monochromatic light source having a wavelength of 555 nanometers (to which the human eye is most sensitive) radiating 1/683 Watts of power in one steradian has a luminous intensity of 1 candela (a steradian is the cone of light spreading out from the source that would illuminate one square meter of the inner surface of a sphere of 1 meter radius around the source). The luminous intensity of a light source in candelas therefore represents a particular direction of light emission (i.e., a light source can be emitting with a luminous intensity of one candela in each of multiple directions, or one candela in merely one relatively narrow beam in a given direction).
From the definition above, it may be appreciated that the luminous intensity of a light source is independent of the distance at which the light emission ultimately is observed and, hence, the apparent size of the source to an observer. Accordingly, luminous intensity in candelas itself is not necessarily representative of the perceived strength of the visual stimulus; rather, a measure of the perceived strength of a visual stimulus that takes into consideration the apparent area of a source from which light is emitted in a given direction is referred to as “luminance,” having units of candelas per square meter (cd/m2). The human eye can detect luminances from as little as one millionth of a cd/m2 up to approximately one million cd/m2 before damage to the eye may occur.
The luminance of a visual stimulus also takes into account the photopic (or scotopic) response of human vision. Recall from the definition of candela above that radiant power is given in terms of a reference wavelength of 555 nanometers. Accordingly, to account for the response of human vision to wavelengths other than 555 nanometers, the luminance of the stimulus (assuming photopic conditions) typically id determined by applying a photopic response function V(λ) to the spectral power distribution (SPD) of the light source giving rise to the stimulus. For example, the luminance L of a given visual stimulus under photopic conditions may be given by:L=K(P1V1+P2V2+P3V3+ . . . ),  (1)where P1, P2, P3, etc., are points on the SPD indicating the amount of power per small constant-width wavelength interval throughout the visible spectrum, V1, V2, and V3, etc., are the values of the V(λ) function at the central wavelength of each interval, and K is a constant. If K is set to a value of 683 and P is the radiance in watts per steradian per square meter, then L represents luminance in units of candelas per square meter (cd/m2).
The “chromaticity” of a given visual stimulus refers generally to the perceived color of the stimulus. A “spectral” color is often considered as a perceived color that can be correlated with a specific wavelength of light. The perception of a visual stimulus having multiple wavelengths, however, generally is more complicated; for example, in human vision it is found that many different combinations of light wavelengths can produce the same perception of color.
Chromaticity is sometimes described in terms of two properties, namely, “hue” and “saturation.” Hue generally refers to the overall category of perceivable color of the stimulus (e.g., purple, blue, green, yellow, orange, red), whereas saturation generally refers to the degree of white which is mixed with a perceivable color. For example, pink may be thought of as having the same hue as red, but being less saturated. Stated differently, a fully saturated hue is one with no mixture of white. Accordingly, a “spectral hue” (consisting of only one wavelength, e.g., spectral red or spectral blue) by definition is fully saturated. However, one can have a fully saturated hue without having a spectral hue (consider a fully saturated magenta, which is a combination of two spectral hues, i.e., red and blue).
A “color model” that describes a given visual stimulus may be defined in terms based on, or related to, luminance (perceived strength or brightness) and chromaticity (hue and saturation). Color models (sometimes referred to alternatively as color systems or color spaces) can be described in a variety of manners to provide a construct for categorizing visual stimuli as well as communicating information to and from color devices regarding different colors. Some examples of conventional color models employed in the relevant arts include the RGB (red, green, blue) model, the CMY (cyan, magenta, yellow) model, the HSI (hue, saturation, intensity) model, the YIQ (luminance, in-phase, quadrature) model, the Munsell system, the Natural Color System (NCS), the DIN system, the Coloroid System, the Optical Society of America (OSA) system, the Hunter Lab system, the Ostwald system, and various CIE coordinate systems in two and three dimensions (e.g., CIE x,y; CIE u′,v′; CIELUV, CIELAB). For purposes of illustrating an exemplary color system, the CIE x,y coordinate system is discussed in detail below. It should be appreciated, however, that the concepts disclosed herein generally are applicable to any of a variety of constructs used to describe a color model, space, or system.
One example of a commonly used model for expressing color is illustrated by the CIE chromaticity diagram shown in FIG. 1, and is based on the CIE color system. In one implementation, the CIE system characterizes a given visual stimulus by a luminance parameter Y and two chromaticity coordinates x and y that specify a particular point on the chromaticity diagram shown in FIG. 1. The CIE system parameters Y, x and y are based on the SPD of the stimulus, and also take into consideration various color sensitivity functions which correlate generally with the response of the human eye.
More specifically, colors perceived during photopic response essentially are a function of three variables, corresponding generally to the three different types of cone receptors in the human eye. Hence, the evaluation of color from SPD may employ three different spectral weighting functions, each generally corresponding to one of the three different types of cone receptors. These three functions are referred to commonly as “color matching functions,” and in the CIE systems these color matching functions typically are denoted as x(λ), y(λ), z(λ). Each of the color matching functions x(λ), y(λ), z(λ) may be applied individually to the SPD of a visual stimulus in question, in a manner similar to that discussed above in Eq. (1) above (in which the respective components V1, V2, V3 . . . of V(λ) are substituted by corresponding components of a given color matching function), to generate three corresponding CIE “primaries” or “tristimulus values,” commonly denoted as X, Y, and Z.
As mentioned above, the tristimulus value Y is taken to represent luminance in the CIE system and hence is commonly referred to as the luminance parameter (the color matching function y(λ) is intentionally defined to match the photopic response function V(λ), such that the CIE tristimulus value Y=L, pursuant to Eq. (1) above). Although the value Y correlates with luminance, the CIE tristimulus values X and Z do not substantially correlate with any perceivable attributes of the stimulus. However, in the CIE system, important color attributes are related to the relative magnitudes of the tristimulus values, which are transformed into “chromaticity coordinates” x, y, and z based on normalization of the tristimulus values as follows:x=X/(X+Y+Z)y=y/(X+Y+Z)z=Z/(X+Y+Z).Based on the normalization above, clearly x+y+z=1, so that only two of the chromaticity coordinates are actually required to specify the results of mapping an SPD to the CIE system.
In the CIE chromaticity diagram shown in FIG. 1, the chromaticity coordinate x is plotted along the horizontal axis, while the chromaticity coordinate y is plotted along the vertical axis. The chromaticity coordinates x and y depend only on hue and saturation, and are independent of the amount of luminous energy in the stimulus; stated differently, perceived colors with the same chromaticity, but different luminance, all map to the same point x,y on the CIE chromaticity diagram. The curved line 50 in the diagram of FIG. 1 serving as the upper perimeter of the enclosed area indicates all of the spectral colors (pure wavelengths) and is often referred to as the “spectral locus” (the wavelengths along the curve are indicated in nanometers). Again, the colors falling on the line 50 are by definition fully saturated colors. The straight line 52 at the bottom of the enclosed area in the diagram, connecting the blue (approximately 420 nanometers) and red (approximately 700 nanometers) ends, is referred to as the “purple boundary” or the “line of purples.” This line represents colors that cannot be produced by any single wavelength of light; however, a point along the purple boundary nonetheless may be considered to represent a fully saturated color. The area bounded by the spectral locus 50 and the purple boundary 52 represents the full “color gamut” of human vision.
In FIG. 1, an “achromatic point” E is indicated at the coordinates x=y=⅓, representing full spectrum white. Hence, colors generally are deemed to become less saturated as one moves from the boundaries of the enclosed area toward the point E. FIG. 2 provides another illustration of the chromaticity diagram shown in FIG. 1, in which approximate color regions are indicated for general reference, including a region around the achromatic point E corresponding to generally perceived white light.
White light often is discussed in terms of “color temperature” rather than “color;” the term “color temperature” essentially refers to a particular subtle color content or shade (e.g., reddish, bluish) of white light. The color temperature of a given white light visual stimulus conventionally is characterized according to the temperature in degrees Kelvin (K) of a black body radiator that radiates essentially the same spectrum as the white light visual stimulus in question. Black body radiator color temperatures fall within a range of from approximately 700 degrees K (generally considered the first visible to the human eye) to over 10,000 degrees K; white light typically is perceived at color temperatures above 1500-2000 degrees K. Lower color temperatures generally indicate white light having a more significant red component or a “warmer feel,” while higher color temperatures generally indicate white light having a more significant blue component or a “cooler feel.”
FIG. 3 shows a lower portion of the chromaticity diagram of FIG. 2, onto which is mapped a “white light/black body curve” 54, illustrating representative CIE coordinates of a black body radiator and the corresponding color temperatures. As can be seen in FIG. 3, a significant portion of the white light/black body curve 54 (from about 2800 degrees K to well above 10,000 degrees K) falls within the region of the CIE diagram generally identified as corresponding to white light (the achromatic point E corresponds approximately to a color temperature of 5500 degrees K). As discussed above, color temperatures below about 2800 degrees K fall into regions of the CIE diagram that typically are associated with “warmer” white light (i.e., moving from yellow to orange to red).
A lighting unit may be configured to generate variable color light or variable color temperature white light based on additive mixing of multiple sources having respective different spectrums. Such a lighting unit may be evaluated in terms of its color generation capability (i.e., an overall range of colors that may be generated) via any one of a variety of color models/spaces/systems. As discussed above in connection with FIG. 1, the CIE color system provides one conventional example of a useful construct for categorizing color, via the CIE chromaticity diagram for example. While the discussion below continues to rely on the CIE color system (and, in particular, the CIE chromaticity diagram) as a construct for evaluating color generation capability of a lighting unit, again it should be appreciated that the concepts disclosed herein generally are applicable to any of a variety of other color models, spaces, or systems that may be employed to evaluate the color generation capability of one or more lighting units.
To illustrate the concept of evaluating a lighting unit for color generation capability, consider a lighting unit that is configured to generate and mix red light, green light, and blue light in various combinations and proportions to create different colors or color temperatures of light (as discussed above, the colors red, green and blue are perhaps somewhat natural choices as they represent “primary” colors of human vision). In terms of the CIE color system, each different source spectrum of the lighting unit (i.e., each of the red light, the green light and the blue light) may be mapped to a corresponding point on the CIE chromaticity diagram, thereby determining a region of the diagram that specifies all of the possible colors (or color temperatures) that may be generated by the lighting unit via additive mixing.
To this end, first a spectral power distribution (SPD) may be measured or estimated (e.g., based on one or more of an expected/approximate dominant wavelength, bandwidth, and radiant power) for each of the different source spectrums of the lighting unit. Typically, the SPDs are measured (or estimated) at maximum available radiant powers for the respective source spectrums. Thus, a red SPD, a green SPD and a blue SPD are obtained, each at maximum available power. Subsequently, CIE chromaticity coordinates x,y are calculated for each SPD in the manner described above in connection with FIG. 1 (i.e., using the color matching functions to obtain tristimulus values X, Y, and Z, and then normalizing), and the calculated coordinates are plotted as points on the CIE chromaticity diagram.
FIG. 4 illustrates the CIE chromaticity diagram of FIG. 1, onto which are mapped exemplary x,y chromaticity coordinates generally indicative of red, green and blue sources that may be employed in one type of lighting unit. The resulting three points 60R, 60G and 60B form an enclosed area (i.e., triangle) referred to as a color gamut 60, representing the colors that may be generated by the lighting unit using the red, green and blue sources based on additive mixing. In FIG. 4, the white light/black body curve 54 and the achromatic point E also are illustrated; as can be seen, a significant portion of the curve 54 falls within the gamut 60, indicating that the lighting unit under evaluation is capable of generating a variety of color temperatures of white light in addition to a variety of more saturated colors within the gamut 60.
It should be appreciated that the gamut 60 shown in FIG. 4 is determined by the particular red, green and blue light sources employed in the specific lighting unit under evaluation. Stated differently, another lighting unit also employing red, green and blue light sources may not have exactly the same color gamut 60 shown in FIG. 4. One reason for this is that one or more of the red, green and blue light generated in one lighting unit may not have exactly or even substantially the same SPD as the corresponding red, green and blue light generated in another lighting unit.
The foregoing situation may arise because of salient spectral differences between any “same color” light sources in the two fixtures including, but not limited to, intentional spectrum specification differences based on the type of source (wideband vs. narrowband sources, slightly different dominant wavelengths), unintentional manufacturing differences that affect spectrum, different aging and/or thermal properties that affect spectrum, etc. Thus, for example, if the red light from one lighting unit has a first red SPD, and the red light from another lighting unit has a second red SPD different from the first red SPD, the respective red x,y chromaticity coordinates for the two lighting units will be different, resulting in different color gamuts for the two lighting units (the same could be said for different green SPDs and/or different blue SPDs for the two lighting units).
FIG. 5 illustrates this situation, showing a second color gamut 60-1 corresponding to a second lighting unit having red, green and blue sources, plotted together with the color gamut 60 from FIG. 4. From FIG. 5, it can be readily observed that each of the red, green and blue chromaticity coordinates for the gamut 60-1 is notably different than those defining the gamut 60, thereby indicating the slightly different spectrums of the “same color” sources in the two lighting units.